Pressurised solvent extraction (PSE) or Accelerated solvent extraction (ASE) [trademark of Dionex and ThermoFisher Scientific] was introduced in the mid 1990s as an alternative to other extraction methods discussed previously such as Soxhlet or sonication for solid samples.  With PSE the sample is placed in a stainless steel cell through which a solvent is passed utilising high temperatures and pressures.  Two common forms of the PSE apparatus exist, one from Dionex is shown in Figure 1, with variations in cell extraction size and solvent selection.

ASE150_350v2

Figure 1 ASE apparatus. [1]

Another pressurised solvent extraction system, the speed extraktor is available from Buchi, see Figure 2 for the apparatus

SpeedExtraktor

Figure 2 SpeedExtraktor E916 from Buchi

The elevated temperature and pressure typically leads to significantly higher capacity of the extraction solvent to dissolve the target analytes, improves the rate of mass transport and effectiveness of sample wetting and matrix penetration, which improves the desorption of analytes from active sites on, and within, the sample particles.   This can result in reduced solvent volumes and shorter extraction durations.  However, pressurised solvent extractors do suffer from a number of drawbacks.  Firstly, the sample size has to be reduced e.g. cut up or milled to allow it to be added to the pressure vessel.  This is not an issue for environmental samples such as soils and why PSE is often used with environmental samples [2].  Due to the efficient nature of the extraction there is a chance that the extraction could be too vigorous and produce extractables that will not be found as leachables.

ASE may be conducted in two ways:

(1) Dynamic ASE, where the solvent is continuously pumped through the extraction cell.

(2) Static ASE, where the extraction cell is filled with solvent, pressurised for a specified time and then drained to the collection vial.

It is also possible to combine the two or to perform multiple extraction cycles.  The majority of ASE applications (about 75%) have been performed in the static extraction mode. Such systems can reach temperatures up to 200 °C and pressures up to 200 bars and can accommodate cells of various volumes, ASE has a significant advantage over a number of extraction techniques in that binary solvent mixtures can be used either using one weak solvent to carry out pre-extraction with a weak solvent to remove some interfering (less strongly sorbed) compounds before the extraction of the compounds of interest [3,4]. The extract complexity may also be reduced by using a sequential extraction procedure with solvents of increasing solvent strength [5].  Such procedures may also be used to assess the strength of matrix–analyte interactions [6].    A recent paper on the utilisation of ASE in the extractable study of single use systems[7] has shown the speed at which an extraction study can take place.

The selectivity of the extraction or leaching process may be further enhanced through the addition of a matrix retainer to the extraction cell.  Use of alumina as a fat retainer was already suggested in 1996 in a Dionex application note [8].   Since then, many other adsorbents have been used for the same purpose, for example, Florisil, silica gel and diol- and cyanopropyl-silica [9].

The Buchi apparatus differs from the ASE apparatus in that it is a parallel system where up to 6 samples can be analysed at the same time rather than in series.  One has higher throughput, the other higher flexibility.  The ASE system can be left running for extened periods with changes to the solvents used, whilst the Buchi system process up to 6 samples at a time.  The Buchi SpeedExtraktor analysis is shown in Figure 3.  The solvent mixture is transferred into separate extraction cells by a pump at high pressure (<150 bar) and heated (<200 °C). The extracts are collected in vials to either be concentrated or directly for analysis.

extraction-approach

Figure 3 Extraction approach for the Buchi SpeedExtraktor

[1] A Feilden., Update on Undertaking Extractable and Leachable testing ISBN 978-1-84735-455-6

[2]  P. Vazquez-Roig, Y. Picó, Pressurized liquid extraction of organic contaminants in environmental and food samples, Trac. Trends Anal. Chem. 71 (2015) 55–64

[3] J. McKiernan ., J. Anal. At. Spectrom., 14(4), 607–613 (1999).

[4] M. Papagiannopoulos and A. Mellenthin, J. Chromatogr. A, 976(1–2), 345–348 (2002).

[5] M. Bergknut , Environ. Toxicol. Chem., 23(8), 1861–1866 (2004).

[6] H. Schroder, J. Chromatogr. A, 1020(1), 131–151 (2003).

[7]  N Dorival-Garcia et Al Talanta Volume 219, 1 November 2020, 121198

[8] Dionex application note ASE 322, Dionex Corp., Sunnyvale, California, USA (1996).

[9] C. Huie, Anal. Bioanal. Chem., 373(1–2), 23–30 (2002).